High Temperature Cuprate Superconductors and Later Discoveries

Abstract

(Bi-2223 with 3 CuO 2 layers). T c thus increases with the number of CuO 2 layers but not beyond three layers. These layered curates have large anisotropy. The superconductivity is strong in the a-b planes (CuO 2 layers) and weak along the c-axis. Critical parameters B c2 and J c too are high in the a-b plane and low along c-direction. Both the materials, Bi-2223 and YBCO are produced commercially and used for selected applications. The new improved 2G-YBCO wires are coated multilayered thin film conductors produced by employing sophisticated techniques and getting popularity among the community. Two more cuprates with still higher T c and analogous to Bi-system were discovered. A T c = 125 in Tl 2 Ca 2 Ba 2 Cu 3 O x (Ti-2223) and 135 K in Hg 1 Ba 2 Ca 2 Cu 3 O 6+δ (Hg-1223) were reported. These materials were, however, not pursued for commercial production because of the toxicity involved. Superconductivity was also found in 2001 in MgB 2 at 39 K, easily attainable with the use of a cryo-cooler. Besides, the material is quite cheap. MgB 2 has two energy gaps and appears to be a BCS superconductor. It is being produced commercially now. The chapter also reviews all the discoveries that took place in 2008 onwards. Hosano reported superconductivity in iron based oxy-pnictides of the type LaFeAsO(1111) around 25 K. T c in excess of 50 K were reported in Sm and Nd based pnictides. The strategy adopted to enhance T c has been to dope the insulating La 2 O 2 layer suitably whereby a charge, electron/hole is transferred to the Fe 2 As 2 conduction layer. Thus a T c = 38 K was reported in a K-doped (Ba 0.6 K 0.4 ) Fe 2 As 2 compound. Very strange behavior of the appearance, disappearance and re-appearance of superconductivity with increasing pressure was announced in an